Olefinic compounds are a class of hydrocarbon compounds which have at least one double bond of four shared electrons between two carbon atoms. In part as a result of their utility as feeds for producing desirable products, olefin demand to grow, particularly for light olefin such as ethylene, propylene, and butenes.
Light olefin is produced in commercial quantities by steam cracking of hydrocarbon-containing feeds. A steam cracker generally includes a plurality of tubular members (typically referred to as “furnace tubes”) located proximate to one or more fired heaters which heat the outer surface of the furnace tubes. A mixture of the feed and steam is introduced into the heated furnace tubes, and heat transferred from the furnace tube to the mixture results in the conversion of at least a portion of the feed to light olefin by pyrolysis. Conditions in the steam cracker are controlled, typically at a temperature in the range of 750° C. to 900° C., to achieve a fixed, predetermined feed conversion, typically in the range of about 60% to about 80%. Although ethylene is the primary light olefin product of steam cracking, the process can also produce appreciable yields of propylene and butenes, particularly when the steam cracker's feed comprises C5+ hydrocarbon. Since steam cracking process conditions are selected to provide a fixed, predetermined feed conversion, ethylene, propylene and butylene yields are substantially constant. Besides light olefin, steam cracking also produces appreciable yields of molecular hydrogen, methane, ethane, propane, butanes, acetylene, butadiene, and C5+ saturated and unsaturated hydrocarbon, including coke. Steam crackers typically include facilities for recovering light olefin from the steam cracker's effluent.
The amount of steam present in the feed-steam mixture is generally in the range of about 0.2 pounds of steam per pound of hydrocarbon feed (0.09 kg/kg) to 0.7 lbs. of steam per pound of hydrocarbon feed (0.32 kg/kg). Typically, steam pressure is in the range of about 30 lbs. per sq. in. (psig, 207 kPag) to about 80 psig (552 kPag). The presence of steam lessens the amount of coke produced during the pyrolysis by decreasing hydrocarbon partial pressure. Steam in the feed-steam mixture also reacts with coke and coke precursors during the pyrolysis, which further decreases the amount coke produced during the pyrolysis. Even with added steam, however, the pyrolysis produces an appreciable yield of coke and coke precursors, and a portion of the coke accumulates in the furnace tubes.
Accumulating coke leads to both an undesirable pressure-drop increase across the tubes' internal flow path and a decrease in heat transfer to the feed-steam mixture. To overcome these difficulties, at least a portion of accumulated coke is removed from the interior of a tube by switching the tube from pyrolysis mode to decoking mode. During decoking mode, the flow of feed-steam mixture into the tube is terminated, and a flow of decoking fluid is established instead. The decoking fluid, typically comprising air and/or steam, reacts with and removes the accumulated coke. When sufficient coke has been removed, the tube is switched from decoking mode to pyrolysis mode to resume light olefin production. Although periodic decoking mode operation is effective for lessening the amount of accumulated coke, this benefit is obtained at a substantial energy cost. In part to lessen damage to the furnace tubes, e.g., by repeated thermal expansion/contractions, the fired heaters operate not only during pyrolysis mode, but also during decoking mode, even though an appreciable amount of recoverable light olefin is not produced during decoking mode.
In order to increase energy efficiency and improve the yield of light unsaturated hydrocarbon, processes have been developed which carry out the pyrolysis in a regenerative pyrolysis reactor. Such reactors generally include a regenerative thermal mass, e.g., a channeled member comprising refractory. The thermal mass is preheated, and then a flow of the hydrocarbon-containing feed is established through the channel. Heat is transferred from the thermal mass to the hydrocarbon feed, which increases the hydrocarbon feed's temperature and results in conversion of at least a portion of the feed by pyrolysis. The pyrolysis produces a pyrolysis product comprising molecular hydrogen, methane, acetylene, ethylene, and C3+ hydrocarbon, which includes coke and coke precursors. At least a portion of the coke remains in the passages of the thermal mass, and the remainder of the pyrolysis product is conducted away from the reactor as a pyrolysis effluent. Ethylene is typically recovered from the pyrolysis effluent downstream of the reactor. Since the pyrolysis is endothermic, pyrolysis mode operation will eventually cool the thermal mass, e.g., to a temperature at which the pyrolysis reactions terminate. The ability to carry out pyrolysis reactions is restored by regenerating the thermal mass during a heating mode. During heating mode, the flow of hydrocarbon-containing feed to the regenerative pyrolysis reactor is terminated. Flows of oxidant and fuel are established to the reactor, typically in a direction that is the reverse of the feed flow direction, and heat is transferred from combustion of the fuel and oxidant to the thermal mass to reheat the thermal mass. After the reactor is sufficiently reheated, the reactor is switched from heating mode to pyrolysis mode.
U.S. Patent Application Publication No. 2016-176781 discloses increasing ethylene yield from a regenerative pyrolysis reactor by operating the pyrolysis mode in an elongated tubular regenerative pyrolysis reactor at a temperature in the range of 850° C.-1200° C. and a hydrocarbon partial pressure ≥7 psia (48 kPa). The reference (e.g., in its FIG. 1A) discloses controlling the pyrolysis mode for increased ethylene selectivity and decreased selectivity for coke and methane by establishing a sharp thermal gradient in the bulk gas temperature profile between a region of substantially constant temperature at which the pyrolysis can occur and a substantially constant lower temperature at which pyrolysis does not occur. During pyrolysis, the position of the gradient within the tubular reactor moves inward as the reactor cools, i.e., toward the midpoint of the reactor's long axis. The cooled reactor is then switched to heating mode, during which the gradient moves outward, i.e., away from the midpoint of the reactor's long axis. Although utilizing such pyrolysis conditions results in a coke yield that is less than that of steam cracking, some coke does accumulate in the channel. Advantageously, the reference reports that accumulated coke can be oxidized to volatile products such as carbon dioxide during heating mode by combustion using a portion of the oxidant in the oxidant flow. Energy efficiency is increased over steam cracking because (i) heating is not needed during pyrolysis mode and (ii) heat released by coke combustion in passages of the thermal mass during heating mode aids thermal mass regeneration. Although the process is more energy efficient than steam cracking, maintaining a sharp temperature gradient in the bulk gas temperature profile leads to substantially constant ethylene and C3+ hydrocarbon selectivities within the pyrolysis zone during pyrolysis mode. Moreover, since the sharp gradient moves downstream during the pyrolysis, the substantially constant ethylene and C3+ selectivities are maintained along the length of the pyrolysis zone for the duration of pyrolysis mode.
Energy efficient processes are now desired which have flexibility to produce a range of product selectivities along the length of the pyrolysis zone during pyrolysis mode, particularly processes which exhibit appreciable feed conversion without excessive coke yield.